Hepatitis C virus (which may be also referred to as “HCV” hereinafter) is a major causative virus of non-A and non-B hepatitis, which infects mainly via transfusion and sexual contact (Choo et al., Science, Vol. 244: 359-362, 1989). It has been estimated that there are 2,000,000 or more HCV carriers in Japan including those who show no hepatitis symptoms (virus carriers), and there are 170,000,000 or more HCV carriers in the world. The major causes for the increasing number of HCV carriers are the fact that the chronicity rate of hepatitis due to HCV infection is as high as 70% to 80%, and the fact that effective antiviral agents other than interferons do not exist.
Pathological conditions exhibited by half or more of chronic hepatitis C patients will almost certainly go from bad to worse and are known to progress to cirrhosis or cancer of the liver. Hence, it can be said that hepatitis C is a serious infectious disease with a poor prognosis. Therefore, studies concerning the treatment of hepatitis C and the detection of HCV are medically important, and development of new therapies and therapeutic drugs has been desired.
HCV is a single-stranded (+) RNA virus having a genome length of approximately 9.6 kb, in which the genome encodes a precursor protein that is converted into 10 types of virus protein (i.e., Core, E1, E2, p7, NS2, NS3, NS4A, NS4B, NS5A, and NS5B proteins) via post-translational cleavage by host-derived signal peptidase or HCV-derived proteases. HCV is classified into 10 or more genotypes (e.g., 1a, 1b, 2a, 2b, 3a, and 3b) according to phylogenetic analysis of the nucleotide sequences of the genome (Choo et al., Science, 1989, Vol. 244, p. 359-362; Simmonds et al., Hepatology, 1994, Vol. 19, p. 1321-1324; Okamoto et al., J. Gen. Virol., 1992, Vol. 73, p. 73-679; and Mori et al., Biochem. Biophys. Res. Commun., 1992, Vol. 183, p. 334-342).
Recently, it has become known that the effects of interferons vary significantly depending on HCV genotype. It has been revealed that the antiviral action of interferons is exerted with difficulty on HCV of genotype 1a or 1b (Fried et al., N. Engl. J. Med., 2002, Vol. 347, p. 975-982 and Lusida et al., J. Clin. Microbiol., 2001, Vol. 39, p. 3858-3864).
Furthermore, it has become known that the antiviral action of interferons is exerted differently on HCV of genotype 2a and HCV of genotype 2b, on which interferons have relatively good effects. It has been suggested that interferons exert their antiviral action more significantly on HCV of genotype 2a than on HCV of genotype 2b (Murakami et al., Hepatology, 1999, Vol. 30, p. 1045-1053).
An HCV antibody test is known as an HCV diagnostic method by which an anti-HCV antibody in serum is detected using a C100-3 antigen, since the anti-HCV antibody recognizing the NS4 region (C100-3 antigen), which is a non-structural region of HCV, exists at the rate of 70%-80% in the serum of a hepatitis C patient (Choo et al., Science, 1989, Vol. 244, p. 359-362). Also, as variations of this method, a second-generation antibody assay system with detection sensitivity that has been improved using a combination of the C100-3 antigen, a core antigen, and an antigen from the NS3 region and a third-generation antibody assay system also containing an antigen from the NS5 region in addition to the above antigens have been developed. HCV antibody tests using these assay systems have been used (Aucella et al., Blood Purif., 2000, Vol. 18, p. 110-114).
Also, other than the aforementioned HCV antibody tests, an HCV core antigen test (Fabrizi et al., J. Clin. Microbiol., 2005, Vol. 43, p. 414-420) is used for direct measurement of the amount of an HCV core protein in serum and a nucleic acid amplification test (NAT) is used for confirmation of the presence or absence of the HCV genome by a PCR method (Velati et al., Euro Serveill., 2005, Vol. 10, p. 12-14).
However, HCV antibody tests are problematic in that when a subject has experienced HCV infection in the past, the subject would unavoidably test positive for hepatitis C, even after being completely cured. HCV antibody tests are also problematic, since an anti-HCV antibody in blood is detected only when 1 to 3 months have passed after infection. If a test is conducted before such time, HCV cannot be detected and the subject would test negative for hepatitis C.
Also, HCV core antigen tests need treatment to cause the liberation of a core protein by disrupting the envelope using SDS, since the core protein (a target molecule) is present within HCV particles. Depending on treatment time with SDS, the core protein may be denatured or substances inhibiting the antigen-antibody reaction may be liberated, thus effecting detection sensitivity.
Furthermore, even when a subject tests positive for HCV in an HCV antibody test and an HCV core antigen test, it is currently impossible to identify the HCV genotype. To conduct interferon therapy, further tests, such as a nucleic acid amplification test, must be conducted in order to identify the HCV genotype. This is because the antiviral action of interferons significantly differs depending on HCV genotype. Particularly on HCV genotype 1a and HCV genotype 1b, effective antiviral action cannot be exerted, and patients rather suffer from adverse effects of interferon.
Meanwhile, a nucleic acid amplification test is problematic in relation to insufficient preservative quality and stability for test samples, since the test uses serum RNA of a subject as a target molecule. The nucleic acid amplification test also presents various problems, and precautions are necessary in regards to the use of an RT-PCR method. For example, PCR may be carried out after transcription of RNA as a target molecule to DNA, resulting in a false negative result due to RNA degradation or inactivation and/or inhibition of a reverse transcriptase or a false positive result due to cross contamination of a reaction system. Hence, the nucleic acid amplification test is thought to be inferior to an HCV antibody test or an HCV core antigen test using a protein as a target molecule in terms of accuracy.